Typical ranging systems in present use employ radar techniques wherein a unique pulse is transmitted toward and reflected from a target or the like whose range is to be determined. The time interval between transmission of the pulse and its reflected return to the transmitter station is indicative of the distance between the transmitter station and the reflecting target.
Radar, of course, has grown into a highly-developed technology and is an extemely accurate and reliable means for determining distance between two objects. It requires highly sophisticated and costly equipment whose sole use is for ranging.
Quite often radio communication data links exist between many objects such as aircraft to aircraft, aircraft to ground, station-ground vehicles to ground vehicles. These data links which utilize continuous waves (as opposed to pulses) are used to transmit data back and forth between the objects.
These data links utilize continuous-wave radio signals which operate within a selected baseband to transmit data between objects.
The present invention contemplates use of a relatively small portion of the data-link frequency spectrum or baseband to determine range information. Thus, the present invention provides a ranging capability by utilizing data links and associated communication equipment which already exist between the objects. Since the ranging system of the present invention utilizes only a small portion of the baseband or frequency spectrum of the data link, actual ranging may take place simultaneously with transmission and reception of data on the same channel. More specifically, the present invention contemplates a ranging system wherein three tones or frequencies within a baseband of a continuous-wave communication system are selected which coincide in phase repetitively at constant time intervals which are greater than the time required to transmit and receive a signal from an object. The present invention generates the three tones from a reference oscillator source. A phase-coincidence detector at the transmitter master station detects phase coincidence of the three tones which provides a start pulse to a counter; and when the three tones are re-transmitted back from a remote station, a phase-coincidence detector at the receiver master station provides a stop pulse to the counter. The count in the counter which is a measure of time for the phase-coincident condition or time marker to be transmitted and received is converted to range.